1. Field of the Invention
The present invention relates generally to a catalyst for purifying exhaust gases in automotive exhaust systems and, more particularly, to a three-way catalyst capable of enhancing the purification performance with respect to HC (hydrocarbons) and the like at a low temperature region.
2. Description of Related Art
For use as a catalyst for purifying automotive exhaust gases, a three-way catalyst has long been well known in the art. The three-way catalyst is of a type capable of simultaneously oxidizing CO (carbon monoxide) and HC (hydrocarbons) and reduce NOx (nitrogen oxides). This three-way catalyst generally comprises .gamma.-alumina (.gamma.-Al.sub.2 O.sub.3) carried by a carrier and deposited with, for example, Pt (platinum) and Rh (rhodium) as noble metal active species, and is known to exhibit a relatively high purifying efficiency with exhaust gases which have resulted from burning of an air-fuel mixture of about 14.7 in stoichiometric air-to-fuel ratio.
One example of such three-way catalyst is disclosed in, for example, Japanese Laid-Open Patent Publication (unexamined) No. 58-36634. This catalyst comprises ceria (CeO.sub.2) and at least one of Pt and Pd (palladium) which are carried by a catalyst carrier. The component ceria has an O.sub.2 storage effect such that when the exhaust gases are in a lean condition, the ceria adsorbs O.sub.2 in the exhaust gases, whereas when the exhaust gases are in a rich condition, the adsorbed O.sub.2 is released, so that the ceria can contribute to oxidation and purification of HC and CO. Through such O.sub.2 storage effect can the exhaust gas atmosphere be brought close to the stoichiometric air-to-fuel ratio and, therefore, Pt and/or Pd is enabled to fully exhibit the activity thereof to thereby effectively purify the exhaust gases.
However, such prior art three-way catalyst is subject to heat deterioration at high temperatures because Pt and Pd components as noble metal active species may become alloyed, or individual Pt components or Pd components may become sintered. As a consequence, the activity of Pt and/or Pd components is lowered, with the result that the catalyst will not exhibit any such purification capability as expected at low temperatures.
In this regard, theoretically it may be conceivable that if the total content of noble metal active species in the catalyst is reduced, any heat deterioration due to alloying, sintering or the like is unlikely to occur. As a matter of fact, however, where the total content of such metal is reduced, the absolute purification capability of the catalyst is lowered. This will in no way be acceptable from the standpoint of practical use.